A trephine is a surgical instrument having a cylindrical blade, and particular types of trephine are used in ophthalmic surgery to cut buttons from donor corneal grafts, and also to cut away diseased sections of a patient's cornea. These trephines have ultra-sharp blades so as to reduce the risk of damage to the cornea when cutting.
Typical corneal trephines are known, for example, from U.S. Pat. No. 4,319,575 and U.S. Pat. No. 2,473,968, the disclosures of which are hereby incorporated into the present application by reference.
A more recent development in this area is the Hessburg-Barron vacuum trephine, available from Barren Precision Instruments, LLC. This comprises an outer, generally cylindrical casing with an annular base and an inner, cylindrical tube of slightly smaller diameter than the outer casing and also having an annular base, slightly recessed from the annular base of the outer casing. This allows the base of the casing to be placed on the curved epithelium of a cornea, with the recessed base of the tube also resting on the epithelium as a result of the convex curvature of the cornea. When a vacuum is applied to the cylindrical space between the outer casing and the inner tube, the casing becomes attached to the epithelium by suction, thereby preventing movement between the casing and the cornea. A cylindrical trephine blade is mounted inside the inner tube and provided with a screw mechanism so as to allow the blade to be raised and lowered within the inner tube. A spoked wheel is provided at an end of the trephine remote from the base so as to allow the amount the blade is raised and lowered to be determined by a number of turns or fractions of turns of the spoked wheel.
In use, the trephine is examined under an operating microscope and the spoked wheel is turned until the blade of the trephine is aligned with the base of the inner tube, this being the zero position. The blade is then retracted by turning the spoked wheel anticlockwise so as to ensure that the blade does not touch the cornea when the vacuum trephine assembly is placed on the epithelium with both the base of the casing and the base of the inner tube contacting the epithelial surface of the cornea. If the blade is not sufficiently retracted, a vacuum between the casing, the inner tube and the cornea cannot be obtained.
A vacuum is then applied to the cylindrical space between the casing and the inner tube, for example by using a syringe with a flexible tube connected to the annular space.
Once a good vacuum seal has been obtained and the assembly is fixed to the cornea by suction, the spoked wheel is rotated clockwise until the blade touches the cornea (this will generally be slightly behind the zero position due to the convex curvature of the cornea), and cutting then starts by continuing to rotate the spoked wheel a desired number of turns. In currently available embodiments of the Hessburg-Barron vacuum trephine, each complete revolution of the spoked wheel raises or lowers the blade of the trephine by approximately 0.25 mm relative to the casing and the inner tube. At the desired depth of cut, the vacuum is released by operating the syringe appropriately, and the trephine is then lifted from the patient's eye.
While operation of the vacuum trephine has been described with reference to a living patient, it may also be used to cut a button from a donor corneal graft harvested from a cadaver and mounted on an artificial anterior chamber.
A significant problem associated with existing vacuum trephines is that the zero position of the blade can be difficult to set accurately. This is because the casing and the inner tube are made of surgical grade stainless steel or the like, and are thus opaque. Because the zero position of the blade is when it is level with the base of the inner tube, which is recessed from the base of the casing, it is not possible to use a microscope to get a square-on view of the blade relative to the base (lower end element) of the inner tube. Instead, it is necessary to observe the base of the inner tube and the blade at an angle, which leads to parallax errors.